The present invention relates to a cubic nitride semiconductor device and a fabrication method of the same.
Cubic nitride semiconductors such as gallium nitride, indium nitride, aluminum nitride and the like are ideal as materials for use in a blue semiconductor laser device, a high-speed transistor of high operating temperatures and the like.
A method for fabricating prior art cubic nitride semiconductor devices will be explained in the following.
FIG. 4 shows a fabrication process of a prior art cubic nitride semiconductor device.
First, a GaAs substrate 4 as shown in FIG. 4(a) is placed in a growth reaction furnace and then heated to about 600.degree. C. in an atmosphere of an As compound, thereby eliminating oxides from the surface of the GaAs substrate 4. Next, the atmosphere of the growth reaction furnace is changed from the As compound to a nitrogen compound such as ammonia(NH.sub.3), dimethylhydrazin((CH.sub.3).sub.2 --N.sub.2 --H.sub.2) or the like and then the heat application process follows in succession. This heat application process is generally referred to as "surface nitriding", whereby As atoms on the surface of GaAs substrate 4 are replaced with N atoms thus having a thin film of GaN crystals formed on the surface of GaAs substrate 4. Then, with a Ga compound furnished, a GaN crystal layer 5, which is a cubic nitride semiconductor, is formed on the GaAs substrate 4 as shown in FIG. 4(b).
In place of the GaAs substrate 4, a gallium phosphide(GaP) substrate, silicon(Si) substrate or the like is also used.
According to the prior art method for fabricating cubic nitride semiconductor devices, however, Ga atoms or GaN molecules are left on the substrate surface when As atoms are replaced with N atoms during the process of surface nitriding. These Ga atoms and GaN molecules, showing high surface mobility and because of mobile force exercised in a direction to mitigate a large amount of surface energy created by a lattice mismatch, move readily on the surface of GaAs substrate, with a confirmed result of having facet-like projections and depressions of several tens to hundreds of angstroms in height formed on the surface of GaAs substrate. (Refer to Japanese Journal of Applied Physics, Volume 33 of 1994, pp. 18-22.) Once these projections and depressions have been formed, the surface morphology of GaN crystal layer thereafter is changed for the worse, having hexagonal nitride semiconductors mixed in the GaN crystal layer with a resulting problem of reducing extensively the crystallization of GaN crystal layer.